Bolometric on-chip temperature sensor

ABSTRACT

Disclosed are embodiments of an improved on-chip temperature sensing circuit, based on bolometry, which provides self calibration of the on-chip temperature sensors for ideality and an associated method of sensing temperature at a specific on-chip location. The circuit comprises a temperature sensor, an identical reference sensor with a thermally coupled heater and a comparator. The comparator is adapted to receive and compare the outputs from both the temperature and reference sensors and to drive the heater with current until the outputs match. Based on the current forced into the heater, the temperature rise of the reference sensor can be calculated, which in this state, is equal to that of the temperature sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/381,427filed May 3, 2006, the complete disclosure of which in incorporatedherewith.

BACKGROUND

1. Field of the Invention

The embodiments of the invention generally relate to on-chip temperaturesensors, and more particularly, to an improved on-chip temperaturecircuit based on bolometry.

2. Description of the Related Art

On-chip temperature sensors are used for various purposes in very largescale integrated circuit (VLSI) technology. For example, temperaturesensors are often used to trigger evasive actions to avoid overheatingor for diagnostic purposes. Such thermal sensors can take many forms.For example, resistors, diodes, or any other temperature sensitiveelements can be used as thermal sensors. Typically, pn junction diodeshave been used because of the nearly ideal behavior of the forwardconduction state in such pn junction diodes. That is, j=j0 exp(Vf Qe/(nk T)), n˜1. However, in silicon-on-insulator (SOI) technology, n istypically a few percent above unity and may also have a significantprocess tolerance. As a result, extra calibration measurements may berequired in order to use this technique for temperature measurement inconjunction with SOI technology. Such extra calibration measurementsincrease the cost of implementation and, thus, limit the use of thistemperature sensing technique. Therefore, there is a need in the art foran improved on-chip temperature sensor and, particularly, for animproved on-chip sensor suitable for use in SOI technology.

SUMMARY

In view of the foregoing, disclosed are embodiments of an improvedon-chip temperature sensing circuit, based on bolometry, which providesself calibration of the on-chip temperature sensors for ideality and anassociated method of sensing temperature at a specific on-chip location.Embodiments of the circuit comprise a temperature sensor, an identicalreference sensor with a thermally coupled heater and a comparator. Thecomparator is adapted to receive and compare the outputs from both thetemperature and reference sensors and to drive the heater with currentsufficient for the outputs to match. Based on the current forced intothe heater, the temperature rise of the reference sensor can becalculated, which in this state, is equal to that of the temperaturesensor.

More particularly, disclosed herein are embodiments of an on-chiptemperature sensing circuit that comprises a comparator (e.g., anoperational amplifier), at least one temperature sensor (i.e., at leastone first temperature sensor), a reference sensor (i.e., a secondtemperature sensor), a heater that is thermally coupled to the referencesensor and driven by the output current from the comparator.

In all embodiments of the invention, the temperature and referencesensors of the circuit should be identical and can comprise any suitabletemperature sensitive element. That is, the temperature and referencesensors should comprise structures adapted to produce outputs (i.e.,first and second outputs, respectively) that are temperature-sensitive.For example, the sensors can comprise thermistors or temperaturesensitive diodes.

In one embodiment of the invention, the circuit comprises a singletemperature sensor and a single reference sensor, each of which iselectrically connected directly to the comparator. In another embodimentof the invention, the circuit comprises multiple temperature sensors anda single reference sensor. The reference sensor is connected directly tothe comparator. The multiple temperature sensors are electricallyconnected to a multiplexer, which is adapted to selectively connect thetemperature sensors to the comparator one at a time. Thus, in each ofthese embodiments the comparator is adapted to receive and compare theoutputs transmitted from a single temperature sensor (i.e., a firsttemperature sensor) and a single reference sensor (i.e., a secondtemperature sensor).

As mentioned above, the circuit comprises a heater that is thermallycoupled to the reference sensor and is powered by the output currentfrom the comparator. An exemplary heater can comprise a diffusedsemiconductor mesa on a dielectric layer above a wafer substrate. Forexample, the heater can comprise an N+ or P+ doped silicon mesa abovethe buried oxide layer of a silicon-on-insulator (SOI) or bulk wafer.The reference sensor can be embedded in the diffused silicon mesa sothat it is thermally coupled to the heater. Electrodes can connect toopposing sides of the diffused silicon mesa so that the mesa can receivethe output current from the comparator and, specifically, so that thecurrent can be passed through the heater and raise the temperature atthe reference sensor. Isolation structures can surround the sidewalls ofthe diffused mesa to electrically isolate the heater from other featuresor devices that are also positioned above the dielectric layer.Similarly, isolation structures can surround the sidewalls of theembedded reference sensor to electrically isolate the reference sensorfrom the current flowing through the heater. Additionally, in order toensure that the heater works with predictable and reproduciblejoule-heating characteristics in SOI technology (i.e., to ensure thatthe heater works independent of process variations), the diffusedsilicon mesa can be formed such that its length and width are eachsignificantly less than the thickness of the substrate but greater thanthe thickness of the buried oxide layer.

Another exemplary heater can comprise a diffused polysilicon mesa on asemiconductor layer above a dielectric layer and wafer substrate. Forexample, the heater can comprise an N+ or P+ doped polysilicon mesaimmediately above a silicon layer on a buried oxide layer of asilicon-on-insulator (SOI) or bulk wafer. The reference sensor can beembedded within the silicon layer directly below the polysilicon mesa sothat it is thermally coupled to the heater. Thermal coupling can beenhanced if the polysilicon mesa overlaps diffused silicon in thesilicon layer adjacent to the reference sensor. Electrodes can connectto opposing sides of the polysilicon mesa so that the mesa can receivethe output current from the comparator and, specifically, so that thecurrent can be passed through the heater and raise the temperature atthe reference sensor. Isolation structures can surround the sidewalls ofthe polysilicon mesa to electrically isolate the heater. Similarly,isolation structures can surround the sidewalls of the embeddedreference sensor to electrically isolate the reference sensor from thecurrent flowing through the heater. Additionally, in order to ensurethat the heater works with predictable and reproducible joule-heatingcharacteristics in SOI technology (i.e., to ensure that the heater worksindependent of process variations), the diffused polysilicon mesa can beformed such that its length and width are each less than the thicknessof the substrate but greater than the thickness of the buried oxidelayer.

In all embodiments of the invention, the comparator is also electricallyconnected to heater and drives the heater until the sensor outputs match(i.e., until the output of the reference sensor is equal to the outputof the temperature sensor). Additionally, the circuit can furthercomprise a register that is adapted to record the amount of currentrequired to drive the heater so that second output equals the firstoutput. Based on the recorded amount of current forced into the heater,the temperature rise of the reference sensor can be calculated, which inthis state, is equal to that of the temperature sensor.

Also disclosed are embodiments of a method of determining a temperature(i.e., a first temperature) at a specified on-chip location (i.e., afirst location) by using an on-chip temperature sensing circuit. Anembodiment of the method comprises forming the on-chip temperaturesensing circuit, as described above. Specifically, the circuit can beformed so that the temperature at a reference sensor (i.e., a secondtemperature) can be raised independent of process variations. This canbe accomplished, for example, by forming a heater with a diffusedsemiconductor mesa above a buried oxide layer and a substrate of a chip.If the length and width of the semiconductor mesa are formed so thatthey are each less than the thickness of the substrate and are eachgreater the thickness of the buried oxide layer, then the temperature atthe reference sensor will be independent of the process variations.

Once the circuit is formed, it can be calibrated. More specifically, ifthe circuit is formed such that the second temperature can be raisedindependent of process variations, then the circuit only needs to becalibrated one time. Calibrating the circuit can be accomplished byheating the entire chip and measuring the output of the reference sensor(i.e., a first value for the second output is determined). After thechip has cooled, power is applied to the heater to raise the temperaturejust at the reference sensor. Then, the output of the reference sensoris again measured (i.e., a second value for the second output isdetermined). A temperature rise in response to the power applied to theheater can be calibrated based on the first and second values.

Once the circuit is calibrated, the first output of the temperaturesensor (i.e., the first temperature sensor) at a first location on thechip can be compared by the comparator to the second output of thereference sensor (i.e., the second temperature sensor). Then, thetemperature at the reference sensor (i.e., the second temperature at thesecond location) is raised (e.g., by applying power to the heater and,specifically, by directing the output current of the comparator into theheater) until the first output equals the second output. The amount ofpower required to raise the second temperature until the first andsecond outputs are equal can be determined by measuring the currentinput into the heater. Based on this amount of power required, thetemperature at the first temperature sensor can be determined.

These and other aspects of the embodiments of the invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following descriptions are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the embodiments of the invention withoutdeparting from the spirit thereof, and the embodiments of the inventioninclude all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be better understood from thefollowing detailed description with reference to the drawings, in which:

FIG. 1 is a schematic diagram illustrating an embodiment of thetemperature sensing circuit of the invention;

FIG. 2 is a schematic diagram illustrating another embodiment of thetemperature sensing circuit of the invention;

FIG. 3 is a diagram illustrating an exemplary heater/reference sensorstructure suitable for incorporation into the temperature sensingcircuits of FIG. 1 and FIG. 2;

FIG. 4 is a diagram illustrating a top view of the structure of FIG. 3;

FIG. 5 is a diagram illustrating a variation on the structure of FIG. 3;

FIG. 6 is a diagram illustrating another exemplary heater/referencesensor structure suitable for incorporation into the temperature sensingcircuits of FIG. 1 and FIG. 2;

FIG. 7 is a diagram illustrating a top view of the structure of FIG. 6;

FIG. 8 is a diagram illustrating a variation on the structure of FIG. 6;and

FIG. 9 is a flow diagram illustrating an embodiment of the method of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments that are illustrated in the accompanyingdrawings and detailed in the following description. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale. Descriptions of well-known components and processingtechniques are omitted so as to not unnecessarily obscure theembodiments of the invention. The examples used herein are intendedmerely to facilitate an understanding of ways in which the embodimentsof the invention may be practiced and to further enable those of skillin the art to practice the embodiments of the invention. Accordingly,the examples should not be construed as limiting the scope of theembodiments of the invention.

As mentioned above, with pn junction diode temperature sensors insilicon-on-insulator (SOI) technology, n is typically a few percentabove unity and may also have a significant process tolerance. As aresult, extra calibration measurements may be required in order to usethis technique for temperature measurement in conjunction with SOItechnology. Such extra calibration measurements increase the cost ofimplementation and, thus, limit the use of this temperature sensingtechnique. Therefore, there is a need in the art for an improved on-chiptemperature sensor and, particularly, for an improved on-chip sensorsuitable for use in SOI technology.

In view of the foregoing, disclosed are embodiments of an improvedon-chip temperature sensing circuit, based on bolometry, which providesself calibration of the on-chip temperature sensors and an associatedmethod of sensing temperature at a specific on-chip location. Thecircuit comprises a temperature sensor, an identical reference sensorwith a thermally coupled heater and a comparator. The comparator isadapted to receive and compare the outputs from both the temperature andreference sensors and to drive the heater with current until the outputsmatch. Based on the current forced into the heater, the temperature riseof the reference sensor can be calculated, which in this state, is equalto that of the temperature sensor.

More particularly, disclosed herein and illustrated in FIGS. 1 and 2,are embodiments of an on-chip temperature sensing circuit (see circuit100 of FIG. 1 and circuit 200 of FIG. 2). Each of these circuits 100 and200 comprise a comparator 101, 201 (e.g., an operational amplifier), atleast one temperature sensor 102, 202 a-d (i.e., at least one firsttemperature sensor), first current source(s) 106 a, 206 a-d to bias thefirst temperature sensor(s), a reference sensor 103, 203 (i.e., a secondtemperature sensor), a second current source 106 b, 206 e to bias thereference sensor 103, 203, a heater 105, 205 that is thermally coupledto the reference sensor 103, 203 and driven by the output current 114,214 from the comparator 101, 201. In one embodiment further elements111, 211, comprising nFETs, form the output circuit of the comparator101, 201 to ensure unidirectional current in the heater, 105, 205. Thisis to avoid a potential instability in the feedback loop.

In all embodiments of the invention, the temperature sensor(s) 102, 202a-d and reference sensor 103, 203 of the circuit 100, 200 should beidentical and can comprise any suitable temperature sensitive element.That is, the temperature and reference sensors should comprisestructures adapted to produce outputs (i.e., first outputs 112, 212 andsecond outputs 113, 213, respectively) that are temperature-sensitive.Thus, those skilled in the art will recognize that while the sensorsillustrated in the circuit diagrams of FIGS. 1 and 2 are diodes (e.g.,pn junction diodes), other suitable temperature sensitive elements, suchas thermistors, bipolar transistors, or FETs, may also be used.

Referring particularly to FIG. 1, in one embodiment of the invention,the circuit 100 comprises a single temperature sensor 102 and a singlereference sensor 103, both of which are electrically connected directlyto the comparator 101. Referring particularly to FIG. 2, in anotherembodiment of the invention, the circuit 200 comprises multipletemperature sensors 202 a-d and a single reference sensor 203. Thereference sensor 203 is connected directly to the comparator 201. Themultiple temperature sensors 202 a-d are electrically connected to amultiplexer 210, which is adapted to selectively connect the temperaturesensors 202 a-d to the comparator 201 one at a time. Thus, in each ofthese embodiments the comparator 101, 201 is adapted to receive andcompare the outputs transmitted from a single temperature sensor (i.e.,a first output 112, 212 of a first temperature sensor 102, 202) and asingle reference sensor (i.e., a second output 113, 213 of the secondtemperature sensor 103, 203). For example, if the temperature andreference sensors are pn junction diodes, then the comparator can beadapted to compare the forward bias voltages of these pn junctiondiodes.

As mentioned above, the circuit 100, 200 comprises a heater 105, 205that is thermally coupled to the reference sensor 103, 203 and ispowered by the current source 111, 211, which in turn comprises theoutput stage 114, 214 of the comparator 101, 201

FIGS. 3 and 4 are side and top view diagrams, respectively, illustratingan exemplary heater/reference sensor structure 300 suitable forincorporation into the temperature sensing circuits 100 and 200 of FIGS.1 and 2. In this structure 300, the heater 305 can comprise a diffusedsemiconductor mesa 343 on a dielectric layer 330 above a wafer substrate320. For example, the heater can comprise an N+ or P+ doped silicon mesa343 on a buried oxide layer 330 of a silicon-on-insulator (SOI) or bulkwafer.

The reference sensor 370 can be embedded in the diffused silicon mesa343 above the buried oxide layer 330 so that it is thermally coupled tothe heater 305. As illustrated, the reference sensor 370 comprises a pnjunction diode that comprises a P+ diffusion region 371 adjacent to N+diffusion regions 372 formed within the diffused silicon mesa 343 suchthat it is surrounded by the heater 305. However, as mentioned above, itis anticipated that the reference sensor 370 can comprise any otherthermally sensitive element, such as a thermistor, bipolar transistor,or FET.

It should be noted that a protective layer 391 can be formed above thediffused silicon mesa 343 and within the reference sensor 370 to preventsilicide formation on exposed silicon, during subsequent processing.This protective layer 391 can comprise either a polysilicon layer abovea thin dielectric or an insulator layer (e.g., a silicon nitride layer).

Electrodes 360 can connect to opposing sides of the heater 305 andspecifically, to opposing sides of the diffused silicon mesa 343. Theseelectrodes 360 allow the heater 305 to receive the output current 390from the comparator and, specifically, allow the current 390 to passthrough the diffused silicon mesa 343 and raise the temperature at thereference sensor 370. If the reference sensor 370 is a pn junctiondiode, as illustrated, then raising the temperature at the referencesensor 370 will effectively reduce the resistance through the diode,thereby, decreasing the forward bias voltage output. Contrarily, if thereference sensor is a thermistor (not shown), then an increase intemperature at the reference sensor will increase the resistance,thereby, increasing the output voltage.

Isolation structures 341 (e.g., oxide or nitride filled shallow trenchisolation (STI) structures) can surround the sidewalls of the diffusedsilicon mesa 343 to electrically isolate the heater 305 from otherfeatures or devices that are also positioned immediately above theburied oxide layer 330. Similarly, referring to FIG. 5, isolationstructures 342 (e.g., oxide or nitride filled STI structures) cansurround the sidewalls of the embedded reference sensor 370 toelectrically isolate the reference sensor 370 from the current 390flowing through the heater 305.

Additionally, in order to ensure that the heater 305 works withpredictable and reproducible joule-heating characteristics in SOItechnology (i.e., to ensure that the heater works independent of processvariations), the diffused silicon mesa 343 can be formed such that itslength 346 and width 345 are each less than the thickness 325 of thesubstrate 620 but greater than the thickness 335 of the buried oxidelayer 330. More specifically, SOI technology, if the active silicon mesa343 has physical length 346 (Lrx) and width 345 (Wrx) much greater thanthe BOX thickness 335 (Tbox), then the thermal conductivity to the bulksubstrate 320 is dominated by the spreading path in the bulk andessentially independent of Tbox. Furthermore, if Lrx 346 and Wrx 345 aremuch smaller than the substrate thickness 325 (Tsx), then the spreadingpath is independent of Tsx and is only a function of Lrx and Wrx. Thus,the heater should be formed such that Tsx>>(Wrx, Lrx)>>Tbox. Forexample, the on-chip structure can be formed such that if Tsx isapproximately equal to 400 um and Tbox is approximately equal to 0.1 um,then Lrx and Wrx of the silicon mesa 343 may be approximately 5 um.

FIGS. 6 and 7 are side and top view diagrams illustrating anotherexemplary heater/reference sensor structure 600 that is also suitablefor incorporation into the temperature sensing circuits 100 and 200 ofFIGS. 1 and 2. In this structure 600, the heater 605 can comprise adiffused semiconductor mesa 653 above a layer 640 comprisingsemiconductor and isolation regions, and dielectric layer 630 on a wafersubstrate 620. For example, the heater 605 can comprise an N+ or P+polysilicon mesa 653 directly above a silicon layer 640 which ispositioned above a buried oxide layer 630 of silicon-on-insulator (SOI)or bulk wafer.

The reference sensor 670 can be embedded within the silicon layer 640directly below the polysilicon mesa 653 so that it is surrounded by and,therefore, thermally coupled to the heater 605. As illustrated, thereference sensor 670 comprises a pn junction diode that comprises a P+diffusion region 671 adjacent to N+ diffusion regions 672 formed withinthe silicon layer. Thermal coupling between the heater 605 and thereference sensor 670 can be enhanced if the polysilicon mesa 653overlaps diffused silicon 647 within the silicon layer 640 adjacent tothe reference sensor 670 so as to allow better transfer of heat to thereference sensor 670. As mentioned above, it is anticipated that thereference sensor 670 can comprise a pn junction diode or any otherthermally sensitive element such as, a thermistor, a bipolar transistor,or a FET.

Electrodes 660 can connect to opposing sides of the polysilicon mesa 653so that the mesa 653 can receive the output current 690 from thecomparator and, specifically, so that the current 690 can pass throughthe heater 605 and raise the temperature at the reference sensor 670. Ifthe reference sensor 670 is a pn junction diode, as illustrated, thenraising the temperature at the reference sensor will effectively reducethe resistance through the diode, thereby, decreasing the forward biasvoltage output. Contrarily, if the reference sensor is a thermistor (notshown), then an increase in temperature at the reference sensor willincrease the resistance, thereby, increasing the output voltage.

Isolation structures 641 (e.g., oxide or nitride filled STI structures)within the silicon layer 640 can electrically isolate the referencesensor 670 from other structures or devices within that layer 640 andisolation structures 661 can surround the sidewalls of the polysiliconmesa 653 to electrically isolate the heater 605. Additionally, referringto FIG. 8, isolation structures 642 can surround the sidewalls of theembedded reference sensor 670 to electrically isolate the referencesensor 670 from the current 690 flowing through the heater 605 and intothe diffusion regions 647.

As with the previously described heater 305, in order to ensure that theheater 605 works with predictable and reproducible joule-heatingcharacteristics in SOI technology (i.e., to ensure that the heater worksindependent of process variations), the diffused polysilicon mesa 653can be formed such that its length 646 and width 645 are each less thanthe thickness 625 of the substrate 620 but greater than the thickness635 of the buried oxide layer 630 (see FIGS. 6 and 7).

Referring again to FIGS. 1 and 2, in all embodiments of the invention,the comparator 101, 201 is also electrically connected to the heater105, 205 so that the output current 114, 214 can drive the heater 105,205 until the sensor outputs 112 and 113, 212 and 213 match (i.e., untilthe output 113, 213 of the reference sensor 103, 203 is equal to theoutput 112, 212 of the temperature sensor 102, 202). The circuits 100,200 may further comprise n-FETs 111, 211 electrically connected betweenthe comparator 101, 201 and the heater 105, 205 to form the outputcircuit of the comparator 101, 201 and, thereby, to ensureunidirectional current in the heater, 105, 205. This is to avoid apotential instability in the feedback loop. Additionally, the circuit100, 200 can further comprise a register 107, 207 that is adapted torecord the amount of current required to drive the heater 105, 205 sothat second output (i.e., the output 113, 213 of the reference sensor103, 203) equals the first output (i.e., the output 112, 212 of thetemperature sensor 102, 202). Based on the recorded amount of currentforced into the heater 105, 205, the temperature rise at the referencesensor 103, 203 can be calculated, which in this state, is equal to thatof the temperature sensor 102, 202.

Thermal sensors often have variability from wafer to wafer and even fromchip to chip. Due to this variability, on-chip thermal sensors oftenrequire extra calibration measurements that greatly increase theimplementation costs. The temperature sensing circuit of the invention,described above, eliminates extra calibration measurements by providinga mechanism for self calibration of the on-chip temperature sensors.More specifically, it is only necessary to calibrate the heater of thecircuit one time. This one-time calibration may be accomplished byeither using simulation (e.g. computer assisted calculation of thethermal heating of the structure in response to the applied power to theheater, using the well-known physics of thermodynamics) or,alternatively, the calibration may be performed empirically, usinghardware. In the latter case, one entire chip is heated to equilibriumusing an external heat source and the output of the reference sensor ismeasured. The calibration of this particular sensor then has knownresponse versus temperature. Next, the chip is allowed to cool, power isrun to the on-chip heater and the output of the reference sensor isagain measured. These measurements, together with the preceding set ofmeasurements, are used to calibrate the temperature rise at thereference sensor versus the power input to the on-chip heater. Becauseof the restrictions on the length and width of the heater, the thermalresponse (temperature rise) of the heater to applied power will benearly independent of process variations, for normal manufacturingtolerances, and this one-time calibration can be used for the entireproduction population of integrated circuits.

More particularly, FIG. 9 illustrates embodiments of a method ofdetermining a temperature (i.e., a first temperature) at a specifiedon-chip location (i.e., a first location) by using an on-chiptemperature sensing circuit (902). The method comprises forming anon-chip temperature sensing circuit, e.g., one of the circuits 100, 200(described above and illustrated in FIGS. 1-8) (902-910). Morespecifically, the circuit 100, 200 can be formed so that the temperatureat the reference sensor (i.e., the second temperature) can be raisedindependent of process variations (910). This can be accomplished, forexample, by forming the heater with a diffused semiconductor mesa (e.g.,either a polysilicon mesa on a semiconductor layer (see FIGS. 6 and 8)or silicon mesa (see FIGS. 3 and 5) above a buried oxide layer and asubstrate of a chip. If the length and width of the semiconductor mesaare formed so that they are each less than the thickness of thesubstrate and are each greater the thickness of the buried oxide layer,then the second temperature will be independent of the processvariations.

Once the circuit is formed (902-910), it can be calibrated (912 a-b).More specifically, if the circuit is formed such that the temperature atthe reference sensor (i.e., the second temperature) can be raisedindependent of process variations, then the circuit only needs to becalibrated one time. The circuit can be calibrated either by simulation(912 a) or by using hardware (912 b). Specifically, calibrating thecircuit using hardware (912 b) can be accomplished by heating the entirechip (914) and measuring the output of the reference sensor (i.e., afirst value for the second output is determined) (916). After the chiphas cooled (918), power is applied to the heater to raise thetemperature at the reference sensor (920). Then, the output of thereference sensor is again measured (i.e., a second value for the secondoutput is determined) (922). The temperature rise in response to thepower applied to the heater can be calibrated based on the first andsecond values (924).

Once the circuit is calibrated (912 a-b), the output of the temperaturesensor at a specified location on the chip (i.e., the first output ofthe first temperature sensor at a first on-chip location) can becompared by the comparator to the output of the reference sensor (i.e.,the second output of the second temperature sensor) (928). Then, thetemperature at the reference sensor (i.e., the second temperature at thesecond location) is raised (e.g., by applying power to the heater and,specifically, by inputting the output current from the comparator intothe heater) until the first output equals the second output (930-932).The amount of the power required to raise the second temperature untilthe first and second outputs are equal can be determined by measuringthe current input (934). Based on this amount of power required, thetemperature at the first temperature sensor can be determined (936).

Therefore, disclosed above are embodiments of an improved on-chiptemperature sensing circuit, based on bolometry, which provides selfcalibration of the on-chip temperature sensors and an associated methodof sensing temperature at a specific on-chip location. The circuitcomprises a temperature sensor, an identical reference sensor with athermally coupled heater and a comparator. The comparator is adapted toreceive and compare the outputs from both the temperature and referencesensors and to drive the heater with current until the outputs match.Based on the current forced into the heater, the temperature rise of thereference sensor can be calculated, which in this state, is equal tothat of the temperature sensor. The benefits of this invention includethose stemming from the ability to accurately measure on-chiptemperatures using temperature-sensitive elements which may varysignificantly within the range on normal manufacturing processtolerances. Furthermore, because this invention can be implemented atlow cost, significant savings in volume manufacturing and test costs canbe afforded. Accurate temperature monitoring thus enabled can furtherbenefit in improved circuit operation as pertains to power, speed, andreliability.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept,and, therefore, such adaptations and modifications should and areintended to be comprehended within the meaning and range of equivalentsof the disclosed embodiments. It is to be understood that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, those skilled in the artwill recognize that the embodiments of the invention can be practicedwith modification within the spirit and scope of the appended claims.

1. A method of measuring a first temperature at a first location on achip, said method comprising: forming an on-chip temperature sensingcircuit comprising: a first temperature sensor at said first location onsaid chip, wherein said first temperature sensor is adapted to produce afirst output that is temperature-sensitive; and a second temperaturesensor at a second location on said chip, wherein said secondtemperature sensor is adapted to produce a second output that istemperature-sensitive; comparing said first output to said secondoutput; raising a second temperature at said second location until saidfirst output equals said second output; and determining said firsttemperature based on an amount of power required to raise said secondtemperature.
 2. The method of claim 1, wherein said forming comprisesthermally coupling said second temperature sensor to a heater.
 3. Themethod of claim 2, wherein said raising of said second temperaturecomprises applying power to said heater by inputting a current into saidheater.
 4. The method of claim 3, wherein said amount of power isdetermined by measuring said current.
 5. The method of claim 1, whereinsaid forming further comprises forming said temperature sensing circuitsuch that said raising of said second temperature is independent ofprocess variations.
 6. A method of measuring a first temperature at afirst location on a chip, said method comprising: forming an on-chiptemperature sensing circuit comprising: a first temperature sensor atsaid first location on said chip, wherein said first temperature sensoris adapted to produce a first output that is temperature-sensitive; anda second temperature sensor at a second location on said chip, whereinsaid second temperature sensor is adapted to produce a second outputthat is temperature-sensitive; calibrating said on-chip temperaturesensing circuit; comparing said first output to said second output;raising a second temperature at said second location until said firstoutput equals said second output; and determining said first temperaturebased on an amount of power required to raise said second temperature,wherein said forming of said on-chip temperature sensing circuitcomprises thermally coupling said second temperature sensor to a heater,and wherein said calibrating comprises: heating said chip: determining afirst value for said second output; applying power to said heater;determining a second value for said second output; and calibrating atemperature rise in response to said power based on said first value andsaid second value.
 7. The method of claim 6, wherein said calibratingfurther comprises: after said determining of said first value and beforesaid applying of said power to said heater, allowing said chip to cool.8. The method of claim 6, wherein said raising of said secondtemperature comprises applying power to said heater by inputting acurrent into said heater.
 9. The method of claim 6, wherein said amountof power is determined by measuring said current.
 10. The method ofclaim 6, wherein said forming further comprises forming said temperaturesensing circuit such that said raising of said second temperature isindependent of process variations.
 11. The method of claim 10, whereinsaid forming further comprises forming said heater with a diffusedsemiconductor mesa above a buried oxide layer and a substrate of a chip,wherein said semiconductor mesa is formed to have a length and a widththat are each less than a first thickness of said substrate and are eachgreater than a second thickness of said buried oxide layer such thatsaid raising of said second temperature is independent of said processvariations.
 12. The method of claim 6, wherein if said raising of saidsecond temperature is independent of process variations, saidcalibrating comprises calibrating only a single time.